Investigating the impact of patient arm position in an MR‐linac on liver SBRT treatment plans

Purpose The superior soft‐tissue contrast offered by the integrated magnetic resonance imaging (MRI) of the Unity MR‐linac compared to the x‐ray‐based image guidance on conventional linacs potentially allows for liver stereotactic body radiation therapy (SBRT) without the need for implanted markers or other surrogates. On conventional linacs, liver SBRT patients are typically positioned with their arms above their heads (arms‐up) to minimize exposure to healthy tissue. However, the spatial confinement of the MRI‐bore and increased treatment delivery times can make the arms‐up position straining. Therefore, we assessed the plan quality for MR‐linac treatment plans with the patient in the arms‐up and in the arms‐down position. Additionally, we compared the MR‐linac plans with clinically used arms‐up treatment plans made for a conventional linac. Methods Fifteen consecutively treated patients with oligometastatic liver disease were included in this retrospective study. For each patient, a planning computed tomography (CT) with delineations, a diagnostic MRI, and a 3 × 20 Gy dual‐arc volumetric modulated arc therapy (VMAT) plan, which was used to treat the patient in an arms‐up position on the conventional linac, were available. For the MR‐linac, 15‐beam step‐and‐shoot intensity‐modulated radiation therapy (IMRT) plans were created for four patient positioning scenarios: arms‐up, mimicking current clinical practice; arms‐down, with treatment beams avoiding the arms on the entrance side; arms‐through, arms are down but not avoided, and right‐arm‐up; only the right arm is up and the left arm is avoided on the entrance side. Resulting treatment plans were compared. Bonferroni‐corrected two‐sided Wilcoxon signed‐ranks tests were used to assess statistical significance (P